You've heard me preach about how you should NOT be the person who shows up to a Halloween party with no costume. And don't be the person with a t-shirt that says "This is my costume."
BUT... in a pinch... these t-shirts from 80sTees.com are mildly acceptable. And I think they're cool. Or at least they're a jumping off point to accessorize and pull off a kitschy costume that'll get you a nod by an appreciative geek.
6 days 'til Halloween... less if the party is on the weekend. Get on it!